Natural attenuation (NA) technology is increasingly being viewed as a cost-effective alternative to engineered schemes for remediation of contaminated aquifers (Kremer, 1998). Some key difficulties in the use of the approach are the complexity of the processes involved, the experience and technical understanding of landowners, practitioners and responsible parties, and the need for a coherent methodology for evaluating its performance.
Many technical protocols have been developed in recent years to provide guidance for implementing NA. These provide a framework for the investigation and monitoring of contaminated sites where natural attenuation is being considered as a remedial option. However, there are significant differences in the protocols in terms of the evidence required to demonstrate whether NA is appropriate and effective. This is the second of two articles about NA. The first (GE January 2000) reviewed the concepts, philosophy and needs of the UK for a formal protocol. Here the site investigation, sampling and monitoring strategies necessary to demonstrate NA and predict its performance are discussed.
Site investigation and monitoring strategies
The site investigation programme needed to evaluate the performance of NA technology incorporates initial site characterisation and longterm monitoring (Kremer, 1998). Site characterisation for NA is fundamentally different and more comprehensive and than that required for an active remediation scheme because it requires greater understanding of processes affecting the contaminant plume and there is greater emphasis on data collected from within the plume.
This initial monitoring phase should identify the location and extent of contaminant source area(s), the spatial distribution and concentration of contaminants, heterogeneity in the aquifer geological and hydrogeological characteristics and variations in the groundwater hydrochemistry. The objective of the initial site characterisation for NA assessment is to define the baseline conditions. This is necessary to ascertain whether NA is likely to be a viable remediation option and, if so, to provide a reference state from which its performance can be monitored over time. A typical network of observation boreholes used to evaluate NA of contaminant plumes is shown in Figure 1.At a minimum the borehole network needs to include wells that assess unimpacted background groundwater quality upstream of the plume (well A), the contaminant composition in the source area (well B) and groundwater quality along the plume flow path (wells C to G). To correctly delineate the plume, boreholes should also be positioned in the flow path ahead of the plume to define the downgradient extent of contamination (well H) and transverse to the plume to define the lateral extent of contaminant migration (wells I and J). The monitoring borehole network (eg wells A to J) is usually installed in phases since the extent of plume migration is unknown until these wells have been drilled.
A conceptual model should be developed using the data generated by the site investigation. The model is a site-specific representation of the contaminant distribution, hydrogeological conditions and NA processes. It is continually refined as new data becomes available, to ensure the suitability of NA as a remediation strategy. Greater initial efforts in developing a technically defensible site conceptual model will provide increased confidence in the selection of NA technology, with reduced long-term monitoring costs.
There are a variety of approaches to assess the suitability of NA at a contaminated site. This assessment can be generalised, using classification schemes based on the potential for biodegradation under different hydrochemical conditions and hydrogeological settings (Wiedemeier and Pound, 1998). Scoring systems can also be used to establish evidence of NA at, for example, chlorinated solvent-contaminated sites using hydrochemical data (Wiedemeier et al, 1999). These methods are screening tools, designed for preliminary assessment to eliminate sites which are unsuitable for NA technology. Dupont also developed a tiered approach to evaluate the feasibility of demonstrating and using NA at chlorinated solvent-contaminated sites, based on the complexity of the site (Table 1). Once the suitability of a site for NA has been determined, a monitoring programme must be developed to demonstrate the processes and assess their performance.
Long-term monitoring (LTM) is required to assess the behaviour of the contaminant plume over time and to confirm that NA is occurring at rates which are protective of downgradient receptors. Additional wells are likely to be needed for LTM. Their number, location, screened intervals and frequency of sampling depends on contaminant distributions, site stratigraphy, plume velocity, groundwater geochemistry and travel time to sensitive receptors (Barcelona, 1994; Wiedemeier and Haas, 1999).
Sampling and analytical strategies
The type, frequency and analytes (the species that are measured) in samples need to be considered in strategies adopted for the performance assessment of NA technology. Natural attenuation processes are typically evaluated using several lines of evidence, which include documentation of contaminant mass loss at fieldscale and hydrogeochemical data indicating that subsurface conditions favour biodegradation (Wiedemeier et al, 1999). The indicator parameters commonly measured in groundwater samples to document NA at sites contaminated with fuel hydrocarbons are summarised in Table 2. These include the organic contaminants, electron acceptors, electron donors, dissolved gases, inorganic and organic metabolites of degradation processes.
Demonstrating NA requires comparison of relative changes in concentrations of the indicator parameters between boreholes outside the plume, the source area and along the plume flow path. However, the identification of contaminant mass distributions and degradation processes is influenced by the design of the monitoring boreholes. Long well screens (eg 10m) used in conventional monitoring wells will cause averaging of the groundwater sample over the screened interval. If the screened interval samples both the uncontaminated and plume groundwater, this may result in underestimation of contaminant concentrations and chemically incompatible sample compositions (eg presence of O 4). An increased density of monitoring points in the borehole network will improve the delineation of the plume but raises the site investigation and monitoring costs. A cost-effective alternative, which minimises these problems, is the use of multilevel samplers (MLS) fitted with small screens (Thornton et al, 1999). The MLS design shown in Figure 2 has monitoring points spaced at 1m intervals, which are fitted with 50mm screens to provide significantly improved resolution of contaminant mass distribution and process discrimination in the plume.
It may be difficult to detect mass loss at fieldscale for plumes containing very high concentrations of organic contaminants (Thornton et al, 1999). In these cases, assessment of NA may rely on the geochemical data and indirect estimates of contaminant mass loss (Thornton et al, 1998).
In UK aquifers which are calcareous (eg Chalk, Triassic sandstone), naturally anaerobic or have naturally higher concentrations of Cl, H (eg Coal Measures, Lincolnshire Limestone) it may be difficult to deduce the extent of NA. This is because the background groundwater chemistry may be similar to that produced by contaminant degradation processes, or mask the production of inorganic metabolites. Since the performance assessment for NA is particularly dependent on hydrochemical analyses of aerobic and anaerobic groundwater samples, it is essential that appropriate procedures are used to obtain representative samples (American Petroleum Institute, 1998; Thornton et al, 1999).
These procedures include the collection and on-site processing of groundwater samples without exposure to the atmosphere to preserve sample quality (Thornton et al, 1999). Good laboratory analytical QC/QA procedures are meaningless if the results have already been biased by poor sampling techniques used in the field!
Data reduction and analytical strategies
A range of methodologies are available to evaluate the performance of NA. These comprise both qualitative and quantitative approaches.
Visual tests include the use of isopleth (concentration contour) maps which provide a two-dimensional plot of changes in solute concentrations over time. These maps will show the plume size and shape, and a general distribution of indicator species within the plume. However, the detail provided and resolution of solute distributions is determined largely by the network density of the monitoring wells and their screened intervals. Additional visual tests include graphical or regression analyses (Wiedemeier et al, 1999). These allow the long-term behaviour of a plume to be deduced and can provide the necessary inputs for simple transport modelling exercises (Wilson 1998).
However, this analysis is difficult to apply at sites with unknown or complex source histories, and the accuracy of the data is largely dependent on the design of the monitoring well network and correct delineation of the plume centreline. This emphasises the need to adequately characterise the plume source during the initial site investigation, to reliably estimate mass loss and plume behaviour.
Statistical analysis of monitoring well data can be undertaken to evaluate plume behaviour, but this usually requires extensive time series data to be statistically significant (Barcelona, 1994; Wiedemeier et al, 1999). A recent methodology for the field-scale performance assessment of NA has been developed by Thornton et al (1998).
The approach uses site investigation and monitoring well data in a simple box model to calculate a plume-scale mass balance for contaminated aquifers, and can provide estimates of the plume source term and mass loss. The technique is particularly useful for understanding plume status, that is, whether the plume is shrinking, stable or expanding under the present conditions. The existing graphical or regression techniques are difficult to apply to expanding plumes (ASTM, 1998).
Selection of NA for the remediation of contaminated sites requires that its performance will meet the clean-up criteria imposed by regulatory agencies. The assessment of NA requires a more comprehensive site investigation and long-term monitoring commitment than that normally undertaken for engineered remediation schemes. Particular attention needs to be given to the design of monitoring well networks and groundwater sampling programmes, upon which the performance assessment is based. A range of methodologies is available to interpret data obtained from the monitoring programme, and the accuracy of the results needs to be interpreted in light of potential limitations imposed by the data set. The existing protocols for the assessment of NA are a good starting point for evaluating the application of the technology in the UK.
However, this guidance should not be considered a 'recipe-book' approach to site investigation for NA. Also, because it is developed largely from American experiences and regulatory perspective, its potential limitations when applied within the different UK hydrogeological and environmental legislative settings need to be borne in mind. The development of a guidance document for the UK, commissioned by the Environment Agency, and now nearing completion, will recognise these differences and will provide an improved basis for the use of the technology in this country. It will ensure consistency in the approaches used for site investigation, sampling and monitoring of NA, which in turn will lead to more rigorous assessments of sites where this technology may be appropriate.
NNAGS is the Network on Natural Attenuation in Groundwater and Soils. It is hosted by the Groundwater Protection and Restoration Group and funded by EPSRC to promote research in the field. NNAGS runs courses, workshops and an annual conference and is open for all to join by visiting the website: www. shef. ac. uk/~nnags/. Steve Thornton is the Environment Agency-sponsored research fellow on Natural Attenuation and can be contacted for advice and further information.
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